The present invention relates to a corrugated fin with partial offset for a plate-type heat exchanger, of the type defining a main overall direction of corrugation and comprising a number of adjacent rows of corrugations, each row being more or less transverse with respect to the said main overall direction and being offset, in its own longitudinal direction, with respect to the two adjacent rows, each row of corrugations comprising a set of corrugation legs connected alternately by a corrugation crest and a corrugation trough.
Corrugated fins of this type, generally known as xe2x80x9cserrated corrugationsxe2x80x9d, are widely used in brazed-plate heat exchangers, which have the advantage of offering a large heat-exchange area in a relatively small volume, and of being easy to manufacture. In these exchangers, fluid flows may be cocurrent, countercurrent or cross-flow.
FIG. 1 of the appended drawings depicts, in perspective and with partial cutaways, one example of such a heat exchanger, of a conventional structure, to which the invention applies. This may, in particular, be a cryogenic heat exchanger.
The heat exchanger 1 depicted consists of a stack of parallel rectangular plates 2, all identical, which between them define a number of passages for fluids to be placed in an indirect heat-exchange relationship. In the example depicted, these passages are, successively and cyclically, passages 3 for a first fluid, 4 for a second fluid and 5 for a third fluid.
Each passage 3 to 5 is bordered by closure bars 6 which delimit it, leaving inlet/outlet openings 7 free for the corresponding fluid. Placed in each passage are corrugated spacer pieces or corrugated fins 8 which act simultaneously as heat-exchange fins and as spacer pieces between the plates, particularly during the brazing operation, and to avoid any deformation of the plates when pressurized fluids are used, and serve to guide the flow of fluids.
The stack of plates, closure bars and corrugated spacer pieces is generally made of aluminium or aluminium alloy and is assembled in a single operation by furnace brazing.
Fluid inlet/outlet boxes 9, of semicylindrical overall shape, are then welded onto the exchanger body thus produced, to cap the corresponding rows of inlet/outlet openings, and are connected to pipes 109 for conveying and removing the fluids.
There are various types of corrugated spacer pieces 8 in existence. The conventional corrugated spacer piece known as the xe2x80x9cserrated corrugationxe2x80x9d is depicted in FIG. 2.
This serrated corrugation has a main overall direction of corrugation D1 and comprises a great many rows of adjacent corrugations 9, all identical 9A, 9B, 9C etc., oriented in a direction D2 perpendicular to the direction D1.
For the convenience of the description, it will be assumed that, as depicted in FIG. 2, the directions D1 and D2 are horizontal.
Each row of corrugations 3 has a crinkled shape and comprises a great many rectangular corrugation legs 10, each contained in a vertical plane at right angles to the direction D2. With respect to an overall direction F of flow of the fluid in the direction D1 in the passage in question, each leg has a leading edge 11 and a trailing edge 12. The legs are connected alternately along their upper edge by flat and horizontal rectangular corrugation crests 13 and along their lower edge by corrugation troughs 14 which are also rectangular, flat and horizontal.
The rows 9 are offset from one another in the direction D2, in one direction and the other alternately. By terming distance p separating two successive legs 10 as the xe2x80x9cpitchxe2x80x9d (neglecting the thickness e of the thin-sheet material of which the corrugation is made), the offset is p/2.
Thus, each row 9 is connected to the next row 9 by the crests 13, in sections of straight line 15 measuring p/2, and by the troughs 14, in sections of straight line 16 with the same lenght p/2. The planes of offsetting are the vertical planes PAB, PBC, etc., and the planes of offsetting when viewed from above are denoted by 17.
Incidentally, the length of each row 9 in the direction D1 is denoted l, this length being termed the xe2x80x9cserration lengthxe2x80x9d, and the height of the corrugation is denoted h.
In practice, the shapes of the various parts of the corrugations may differ somewhat from the theoretical shapes described hereinabove, particularly as regards the flatness of the facets 10, 13 and 14, the verticality and the rectangular shape of these facets.
FIGS. 3 to 5 of the appended drawings are schematic cross sections taken, respectively, on the vertical plane IIIxe2x80x94III of FIG. 2, approximately on an offsetting plane P and on the horizontal mid-plane Q of the corrugation. These views illustrate the disadvantage of conventional serrated corrugations.
What happens is that a given stream of fluid flowing in the overall direction D1 has available to it, within a row 9, for example 9A, a wide passage cross section (FIG. 3), but this cross section is reduced in each plane P because of the presence of the legs 10 from the next row 9, in this instance the legs 10B of the row 9B.
Thus, the characteristic offsetting of the serrated corrugations introduces a substantial pressure drop. In order to limit this effect, relatively long serration lengths l need to be adopted, although these are not optimum from the thermal efficiency standpoint.
The object of the present invention is to reduce or even to eliminate the pressure drops induced in the serrated corrugations by the offset from one row to the next.
To this end, a subject of the invention is a corrugated fin with partial offset of the aforementioned type, characterized in that at least some corrugation legs have a notch on at least one edge and over at least part of their height.
Another subject of the invention is a plate-type heat exchanger comprising corrugated fins as defined above. This exchanger, of the type comprising a stack of parallel plates which define a number of passages of flat overall shape for the circulation of fluids, closure bars which delimit these passages, and corrugated fins arranged in the passages, is characterized in that at least some of the corrugated fins are according to the definition provided above.